arXiv subjects

Lutz Strassburger

Publications and source records attributed to Lutz Strassburger.

7 recordsLinked to original sources

arXiv2025

Intuitionistic BV (Extended version)

We present the logic IBV, which is an intuitionistic version of BV, in the sense that its restriction to the MLL connectives is exactly IMLL, the intuitionistic version of MLL. For this logic we give a deep inference proof system and show cut elimination. We also show that the logic obtained from IBV by dropping the associativity of the new non-commutative seq-connective is an intuitionistic variant of the recently introduced logic NML. For this logic, called INML, we give a cut-free sequent calculus.

cs.LO↗

arXiv2023

Coqlex: Generating Formally Verified Lexers

A compiler consists of a sequence of phases going from lexical analysis to code generation. Ideally, the formal verification of a compiler should include the formal verification of each component of the tool-chain. An example is the CompCert project, a formally verified C compiler, that comes with associated tools and proofs that allow to formally verify most of those components. However, some components, in particular the lexer, remain unverified. In fact, the lexer of Compcert is generated using OCamllex, a lex-like OCaml lexer generator that produces lexers from a set of regular expressions with associated semantic actions. Even though there exist various approaches, like CakeML or Verbatim++, to write verified lexers, they all have only limited practical applicability. In order to contribute to the end-to-end verification of compilers, we implemented a generator of verified lexers whose usage is similar to OCamllex. Our software, called Coqlex, reads a lexer specification and generates a lexer equipped with a Coq proof of its correctness. It provides a formally verified implementation of most features of standard, unverified lexer generators. The conclusions of our work are two-fold: Firstly, verified lexers gain to follow a user experience similar to lex/flex or OCamllex, with a domain-specific syntax to write lexers comfortably. This introduces a small gap between the written artifact and the verified lexer, but our design minimizes this gap and makes it practical to review the generated lexer. The user remains able to prove further properties of their lexer. Secondly, it is possible to combine simplicity and decent performance. Our implementation approach that uses Brzozowski derivatives is noticeably simpler than the previous work in Verbatim++ that tries to generate a deterministic finite automaton (DFA) ahead of time, and it is also noticeably faster thanks to careful design choices. We wrote several example lexers that suggest that the convenience of using Coqlex is close to that of standard verified generators, in particular, OCamllex. We used Coqlex in an industrial project to implement a verified lexer of Ada. This lexer is part of a tool to optimize safety-critical programs, some of which are very large. This experience confirmed that Coqlex is usable in practice, and in particular that its performance is good enough. Finally, we performed detailed performance comparisons between Coqlex, OCamllex, and Verbatim++. Verbatim++ is the state-of-the-art tool for verified lexers in Coq, and the performance of its lexer was carefully optimized in previous work by Egolf and al. (2022). Our results suggest that Coqlex is two orders of magnitude slower than OCamllex, but two orders of magnitude faster than Verbatim++. Verified compilers and other language-processing tools are becoming important tools for safety-critical or security-critical applications. They provide trust and replace more costly approaches to certification, such as manually reading the generated code. Verified lexers are a missing piece in several Coq-based verified compilers today. Coqlex comes with safety guarantees, and thus shows that it is possible to build formally verified front-ends.

cs.PL↗

arXiv2015

On Nested Sequents for Constructive Modal Logics

We present deductive systems for various modal logics that can be obtained from the constructive variant of the normal modal logic CK by adding combinations of the axioms d, t, b, 4, and 5. This includes the constructive variants of the standard modal logics K4, S4, and S5. We use for our presentation the formalism of nested sequents and give a syntactic proof of cut elimination.

cs.LO↗

arXiv2009

A System of Interaction and Structure IV: The Exponentials and Decomposition

We study a system, called NEL, which is the mixed commutative/non-commutative linear logic BV augmented with linear logic's exponentials. Equivalently, NEL is MELL augmented with the non-commutative self-dual connective seq. In this paper, we show a basic compositionality property of NEL, which we call decomposition. This result leads to a cut-elimination theorem, which is proved in the next paper of this series. To control the induction measure for the theorem, we rely on a novel technique that extracts from NEL proofs the structure of exponentials, into what we call !-?-Flow-Graphs.

cs.LO↗

arXiv2007

On the Axiomatisation of Boolean Categories with and without Medial

The term ``Boolean category'' should be used for describing an object that is to categories what a Boolean algebra is to posets. More specifically, a Boolean category should provide the abstract algebraic structure underlying the proofs in Boolean Logic, in the same sense as a Cartesian closed category captures the proofs in intuitionistic logic and a *-autonomous category captures the proofs in linear logic. However, recent work has shown that there is no canonical axiomatisation of a Boolean category. In this work, we will see a series (with increasing strength) of possible such axiomatisations, all based on the notion of *-autonomous category. We will particularly focus on the medial map, which has its origin in an inference rule in KS, a cut-free deductive system for Boolean logic in the calculus of structures. Finally, we will present a category of proof nets as a particularly well-behaved example of a Boolean category.

cs.LO↗

arXiv2006

Proof Nets and the Identity of Proofs

These are the notes for a 5-lecture-course given at ESSLLI 2006 in Malaga, Spain. The URL of the school is http://esslli2006.lcc.uma.es/ . This version slightly differs from the one which has been distributed at the school because typos have been removed and comments and suggestions by students have been worked in. The course is intended to be introductory. That means no prior knowledge of proof nets is required. However, the student should be familiar with the basics of propositional logic, and should have seen formal proofs in some formal deductive system (e.g., sequent calculus, natural deduction, resolution, tableaux, calculus of structures, Frege-Hilbert-systems, ...). It is probably helpful if the student knows already what cut elimination is, but this is not strictly necessary. In these notes, I will introduce the concept of ``proof nets'' from the viewpoint of the problem of the identity of proofs. I will proceed in a rather informal way. The focus will be more on presenting ideas than on presenting technical details. The goal of the course is to give the student an overview of the theory of proof nets and make the vast amount of literature on the topic easier accessible to the beginner. For introducing the basic concepts of the theory, I will in the first part of the course stick to the unit-free multiplicative fragment of linear logic because of its rather simple notion of proof nets. In the second part of the course we will see proof nets for more sophisticated logics. This is a basic introduction into proof nets from the perspective of the identity of proofs. We discuss how deductive proofs can be translated into proof nets and what a correctness criterion is.

cs.LO↗

arXiv2006

From Proof Nets to the Free *-Autonomous Category

In the first part of this paper we present a theory of proof nets for full multiplicative linear logic, including the two units. It naturally extends the well-known theory of unit-free multiplicative proof nets. A linking is no longer a set of axiom links but a tree in which the axiom links are subtrees. These trees will be identified according to an equivalence relation based on a simple form of graph rewriting. We show the standard results of sequentialization and strong normalization of cut elimination. In the second part of the paper we show that the identifications enforced on proofs are such that the class of two-conclusion proof nets defines the free *-autonomous category.

cs.LO↗